1. Field of Invention
The present invention relates to transmission of an optical signal, in particular to a hybrid electro-optical circuit board that integrates a light-guide device with an electro-optical element.
2. Related Art
Most of the high-speed electronic communication systems perform electrical signal transmission using metal wires. In order to enhance the communication bandwidth, the frequency of the electrical signal has been significantly enhanced. However, the electrical signal running at a high frequency results in severe electromagnetic interference. In order to reduce the electromagnetic interference, the transmission bandwidth and frequency of the electrical signal are limited, thus it is difficult to further increase the data transfer rate in the electrical signal transmission. During the optical signal transmission, neither electromagnetic wave leakage, nor suffered from the interference of the outer electromagnetic wave is caused, and thus being gradually integrated into the electrical circuit, to serve as a channel for signal transmission.
In the current optical signal transmission, an optical fiber is used to transmit an optical signal, and the optical fiber is connected to an electro-optical element through a coupler, so as to convert the optical signal into an electrical signal to be transmitted to other electronic elements. The current approach is to integrate a light-guide device with an electro-optical element onto a circuit board to form a hybrid electro-optical circuit board, for example, as disclosed in U.S. Pat. No. 6,512,861, an electro-optical element for converting an optical signal to an electrical signal is disposed on the upper surface of a plate, and a light-guide device is formed on the lower surface of the plate, and covers a light guiding hole of the plate, so that the optical signal is transmitted between the electro-optical element and an optical waveguide element via the light guiding hole.
However, the above design has disadvantages. Generally, a printed circuit board serving as the plate is formed with resin-glass fibers and metal circuits. In the subsequent process for fabricating the circuit board, chemical substances have to be used for removing the adhesive glue residuals from the light guiding hole, and for etching for the metal circuits, or adhesive glue is required for laminating the elements or multi-layer boards. As a result, during the above two processes, the chemical substances or the adhesive glue enter the light guiding hole. The chemical substances, such as strong bases, for example, potassium hydroxide (KOH), or strong acids, corrode the inner wall of the light guiding hole, such as to roughen the inner wall of the light guiding hole. Therefore, the reflection efficiency of the inner wall of the light guiding hole to the optical signal is degraded, such that the intensity of the optical signal rapidly decays after passing through the light guiding hole. On the other aspect, the adhesive glue fills the light guiding hole, and blocks the traveling path of the optical signal, thus directly affecting the yield of the manufacturing process. Therefore, how to ensure the light guiding hole not to be damaged during the manufacturing process becomes a problem to be solved.